The invention relates generally to fluid gauging apparatus and methods such as used for fuel gauging on aircraft. More particularly, the invention relates to apparatus and methods for inductive interrogation of fluid gauging sensors.
Significant advances have been made in developing new high performance materials such as, for example, graphite/epoxy composites. These materials and many other composite types hold great promise for replacing conventional materials such as steel and aluminum in structures subjected to various environmental conditions such as high temperature, pressure, stress and strain. For example, carbon composite materials are expected to be used extensively in next generation aircraft for structures such as the wings and other air foil surfaces, engine drive shafts and so on just to name a few examples.
Although these materials have been shown to offer substantial benefits over conventional materials, industry acceptance of these materials, especially in the aerospace industry, has been limited due to the lack of statistical databases on their failure modes because of their relatively recent introduction. This situation presents a catch-22 because until such materials have been subjected to extensive use, such databases will remain relatively unavailable. Also, the lack of statistical analysis results in structures using these materials being over designed to the point that the benefits of using the improved materials are reduced or eliminated altogether.
Because of the need to characterize the real-world performance of these materials, extensive activity has been undertaken to develop "smart" structures in which the structures include embedded or integrated sensors that monitor one or more structural parameters such as stress, strain etc. Such smart structures are described, for example, in U.S. Pat. Nos. 4,983,034 and 4,930,852 issued to Spillman and Wheeler et al. respectively, and commonly owned by the assignee of the present invention, the entire disclosures of which are fully incorporated herein by reference. The basic concept is that the integrated sensors can be used as health monitors for the structure to characterize the structural performance of the materials. Having such real time or near real time information can allow structures to be designed up to the material limits. In fact, the structures can further include active elements that provide adaptive compensation for structure performance. Such a smart structure concept is disclosed in U.S. Pat. No. 4,922,096 issued to Brennan and U.S. patent application Ser. No. 07/981,966 filed on Nov. 25, 1992 for "Smart Structure With Non-Contact Power and Data Interface", which are commonly owned by the assignee of the present invention, the entire disclosures of which are fully incorporated herein by reference.
Optical sensors such as those described in the referenced patents are a desirable solution to providing smart structures. Such sensors conveniently use optic fibers as part of the active sensor element, and of course, optic fibers are small and flexible thus making them ideal for embeddment in composite structures without adversely affecting structural integrity. For example, structural strain can be monitored by transmitting light through one or more embedded optic fibers that bend under stress, strain etc. thus affecting the transmission characteristics of the light through the fiber.
The successful use of such smart structures, optics based or otherwise, requires a reliable and accurate way to interrogate the sensors. Optical sensors present a particularly difficult ingress/egress data and power problem because light energy must be delivered to the sensor and the output light pattern or signals corresponding thereto must be coupled back out to the outside world for processing and analysis. Using hardwired or other mechanical, electrical or optical connections between the sensors and external hardware is difficult from a manufacturing stand point due to the need for precise machining and very tight tolerances to allow efficient coupling. Although it is generally known to use RF coupling for contactless interrogation of some sensors such as tire pressure sensors, such approaches typically depend on frequency domain analysis (such as by detecting a resonant frequency shift based on a sensed parameter) and do not provide compensation for the gap between the transmitting and receiving coils. These approaches are not practical, therefore, for resistive sensors, nor for extracting data from optical sensors because these sensors produce amplitude dependent outputs.
In addition to the desirability of being able to characterize the performance of composite materials and to interrogate sensors embedded therein, in some applications a sensor can be contained within a composite enclosure. For example, in a composite fuel tank on an aircraft, there may be one or more fuel quantity sensors. Such sensors can be capacitive fuel level sensors or pressure sensors, to name just two examples. In order to maintain structural integrity of the fuel tank, it is desirable that the sensors be interrogated without hardwire connections or the like, because such an arrangement could compromise the structural integrity due to the need to provide access the sensors.
Accordingly, the objective exists for non-contact apparatus and methods for interrogating smart structure sensors, particularly sensors enclosed in composite structures such as within a fuel tank or other fluid container.